We report the case of a 63-year-old woman who developed cough and fever with migratory lung infiltrates three months after completion of right breast irradiation following conservative surgery. Lung infiltrates were initially localized in the irradiated area, but later spread to unirradiated areas in both lungs. No cause of migratory pneumonitis other than irradiation was found, and we clinically diagnosed this case as radiation-induced migratory pneumonitis similar to BOOP, without lung biopsy. Steroid therapy resulted incomplete resolution of lung infiltrates. The reported case clearly differed from typical radiation pneumonitis. We suggest that lung irradiation might trigger the development of migratory pneumonitis with a clinical pattern similar to that of BOOP.